The invention herein disclosed is related to co-owned U.S. Pat. No. 6,268,948 entitled xe2x80x9cMicromachined Reflective Light Valvexe2x80x9d, which issued on Jul. 31, 2001.
The invention relates to multi-channel recording of modulated light beams and particularly to writing on media of variable surface profiles, such as the patterned surfaces of semiconductor wafers and masks.
Light valve based multi-channel optical recording systems divide a beam of light into a plurality of sub-beams and employ the sub-beams to record data on a recording surface. Each sub-beam corresponds to a particular xe2x80x9cchannelxe2x80x9d on the recording surface. Prior art multi-channel recording systems suffer from several disadvantages which are related to the fact that they use only one optical output system to focus all of the sub-beams. The single optical output system has only one focal length; that is, prior art multi-channel optical recording systems attempt to focus all of the sub-beams to the same depth. This is the case for all prior art multi-beam and/or multi-channel systems, not only ones based on light valves. To provide the best all around focus for the multi-channel system, this focal length is typically selected to be some sort of average over the range of focal lengths required for all of the channels.
Having all of the channels share a single focal length results in a serious deficiency when the number of channels in the recording system is large. In such a situation, the output optics require a relatively large field of view to accommodate the large number of sub-beams. The larger the field of view, the larger the variation required in the optimal focusing point for the various sub-beams. Thus, the maximum number of channels of the optical recording system is limited by the variation in required focal length between the sub-beams.
Additionally, a multi-channel recording system with a single focal length is not suitable to record on surfaces with variable depths or patterned surface profiles, such as the surface of a semiconductor wafer during the lithography process. Clearly, a multi-channel recording system with a single focal length (i.e. a system having all sub-beams focussed at the same depth) can not be used effectively to simultaneously write data to each channel of a surface having regions of variable depth, even if an auto-focus mechanism is used.
Accordingly, it is an object of the present invention to overcome these deficiencies in the prior art by providing a multi-channel optical recording system, wherein the sub-beams of light corresponding to each channel can be individually focussed.
It is a further object of this invention to provide a multi-channel recording that can effectively and efficiently record data into each channel simultaneously on a recording surface having a variable surface profile, such as the patterned surface of a semiconductor wafer during the lithography process.
The invention herein disclosed relates to an apparatus for a multi-channel optical imaging system. The apparatus includes two sub-systems: a focus sensing subsystem and a focus control and recording subsystem. The focus sensing subsystem is operative to detect a depth profile of a recording surface and use the depth profile to determine a desired focal depth for each of a plurality of recording channels. The focus control and recording subsystem is operative to individually focus light containing optical image data to an actual focal depth, such that in each recording channel, the actual focal point substantially matches the desired focal point. The focus control and recording system is also operative to record the optical image data in each of the plurality of recording channels.
Preferably, the apparatus may further comprise a common imaging lens, which is operative to image the light for each of the plurality of channels during recording.
Advantageously, the focus control and recording subsystem may further comprise a focus control light valve. The focus control light valve comprises an array of individually addressable, deformable, reflective ribbons, suspended over a corresponding array of cavities in a substrate. The ribbons are operative to reflect light in a manner so as to generate a plurality of sub-beams of light, each sub-beam corresponding to one of the plurality of recording channels. Upon application of known electric signals in the vicinity of each of the ribbons, the ribbons are also individually operative to deform a variable amount into the cavities. The deformation of the ribbons changes the focus properties of the sub-beams so as to individually control the actual focal depth for each sub-beam. The amount of deformation of the ribbons is controlled by the electrical signals such that the actual focal depth substantially matches the desired focal depth for each of the channels. Preferably, the known electric signals may contain information about the desired focal depth for each of the plurality of channels on the recording surface, as determined by the focus sensing subsystem.
Advantageously, the focus control light valve may be further operative to modulate each of the sub-beams with optical image data in a manner such that each sub-beam is modulated with optical image data for a distinct one of the plurality of channels.
Alternatively, the apparatus may further comprise a modulation subsystem which is distinct from the focus control light valve and is operative to generate a plurality of sub-beams of light. The distinct modulation subsystem modulates the light prior to the light reaching the focus control light valve. The modulation subsystem which is distinct from the focus control light valve may either be an array of individually addressable laser diodes, where each laser diode creates one sub-beam of modulated light, or, alternatively, a modulation light valve similar to the focus control light valve, but independently operative to modulate each of the sub-beams with the optical image data.
Advantageously, the focus sensing subsystem may further comprise a focus sensing light source and a set of optical elements. The set of optical elements may either be distinct from the common imaging lens, or it may include the common imaging lens. The focus sensing light source and the set of optical elements may be operative, in combination, to image a linearly shaped beam of light onto the recording surface. The focus sensing subsystem may also further comprise a photo-detector located at a non-perpendicular angle from the recording surface, such that when the linearly shaped beam of light is reflected from the recording surface, it creates a two dimensional image on the photo-detector. The shape of the two dimensional image may be proportional to the depth profile of the recording surface. Preferably, the photo-detector may be either an array of charged coupled devices, or an array of position sensitive detectors.
Advantageously, the apparatus may be implemented with an plurality of channels where the channels are arranged in either a linear fashion or a two dimensional fashion.
Another aspect of the invention concerns a method of multi-channel optical imaging, which comprises the steps of:
(a) detecting a depth profile of a recording surface;
(b) using the depth profile of the recording surface to determine a desired focal point for each of a plurality of recording channels;
(c) individually focusing light bearing optical image data to an actual focal point in each recording channels by using the desired focal points as a reference such that the actual focal point substantially matches the desired focal point in each recording channel; and
(d) recording the optical image data in each of the plurality of channels on the recording surface.
Advantageously, the recording step may further comprise a step of imaging the light for each of the plurality of channels through a common imaging lens.
The adjusting step may also comprise the additional steps of:
(a) generating a plurality of sub-beams of light, where each sub-beam corresponds to one of the plurality of channels, by reflecting the light from an array of individually addressable, deformable, reflective ribbons, which are suspended over a corresponding array of cavities in a substrate; and
(b) applying known electrical signals selectively to each of the ribbons. The electrical signals cause the ribbons to deform a variable amount into the cavities, and the deformation of the ribbons changes the focus properties of the sub-beams so as to individually control the actual focal point for each sub-beam, such that the actual fecal point substantially matches the desired focal point for each of the plurality of channels.
Preferably, the electric signals contain information about the desired focal depth for each of the plurality of channels on the recording surface, as determined by the determining step.
Advantageously, the applying step may further comprise the step of modulating each of the sub-beams with the optical image data. The modulating step may be effected by the array of individually addressable, deformable, reflective ribbons.
Alternatively, the method may comprise the additional steps of producing a plurality of sub-beams of light, and modulating each of the sub-beams with the optical image data. The producing and modulating steps precede the generating step and are effected by means other than the array of individually addressable, deformable, reflective ribbons.
The detecting step may further comprise the step of imaging a linearly shaped beam of light onto the recording surface by using a light source and a set of optical elements. The set of optical elements may either be distinct from the common imaging lens or it may include the common imaging lens. The detecting step may also further comprise the step of creating a two dimensional image on a photo-detector, where the two dimensional image is a non-perpendicular image of the linearly shaped beam of light, distorted by the recording surface. Advantageously, the shape of the two dimensional image on the detector may be proportional to the depth profile of the recording surface.